Wiring materials such as insulated wires, cables, cords, optical fiber core wires or optical fiber cords (optical fiber cables), used in an electrical and electronic equipment field and an industrial field are required to have various characteristics such as flame retardancy, heat resistance and mechanical characteristics (for example, tensile properties and abrasion resistance).
In addition, these wiring materials are heated to 80 to 105° C., and further to about 125° C., by use for a long period of time, and are required to have heat resistance thereto in several cases. In such a case, a rubber material or a crosslinked material is used as the wiring material. As a method for crosslinking rubber, a chemical crosslinking method is generally employed, and when the crosslinked material is produced, an electron beam crosslinking method or a chemical crosslinking method is employed.
Conventionally, as methods for crosslinking polyolefin resins such as polyethylene, known so far include: electron beam crosslinking methods in which the resin is crosslinked by irradiation with electron beams, and chemical crosslinking methods, such as a crosslinking method in which heat is applied after forming, to decompose organic peroxide or the like and to allow a crosslinking reaction, and a silane crosslinking method.
The silane crosslinking method means a method of obtaining a crosslinked resin, by obtaining a silane-grafted resin by allowing a grafting reaction of a silane coupling agent having an unsaturated group with a resin in the presence of organic peroxide, and then bringing the silane-grafted resin into contact with moisture in the presence of a silanol condensation catalyst.
Among the above-described crosslinking methods, in particular, the silane crosslinking method requires no special facilities in many cases, and therefore can be employed in a wide range of fields.
However, when the silane grafting reaction is performed by using a kneader or a Banbury mixer in the silane crosslinking method, the silane coupling agent having the unsaturated group generally has high volatility and has a problem that it volatilizes before participating in the silane grafting reaction. Therefore, it has been difficult to prepare a desired silane crosslinked master batch containing the silane-grafted resin.
Furthermore, it is more difficult to cause silane crosslinking of a halogen-containing resin such as a polyvinyl chloride resin, chlorinated polyethylene and chloroprene rubber than to cause silane crosslinking of the above-described polyolefin resin, by the silane crosslinking method. It is difficult to produce the crosslinked resin merely by allowing the silane grafting reaction of the silane coupling agent having the unsaturated group with the halogen-containing resin in the presence of organic peroxide, to obtain the silane-grafted resin, and then bringing the silane-grafted resin into contact with moisture in the presence of the silanol condensation catalyst.
To take an example of the silane crosslinking method, for example, Patent Literature 1 proposes a method of sufficiently melt-kneading an inorganic filler subjected to surface treatment with a silane coupling agent, a silane coupling agent, an organic peroxide, and a crosslinking catalyst to a polyolefin-based resin, by a kneader, and then forming the resultant material by a single screw extruder.
In addition, Patent Literatures 2 to 4 propose a method of partially crosslinking a vinyl aromatic thermoplastic elastomer composition prepared by applying a block copolymer or the like as a base polymer and adding a softener for non-aromatic-rubber as the softener thereto, by using an organic peroxide, through an inorganic filler subjected to surface treatment with silane.
Patent Literature 5 proposes a method of obtaining a cable having heat resistance, by adding an organic peroxide, and a silane coupling agent, to a base material, together with an inorganic filler, and further extruding the resultant material together with a silanol condensation catalyst, and then bringing the resultant material into contact with moisture.